1. Industrial Field of the Invention
The present invention relates to a method for pulling a silicon single crystal according to the Czochralski process, and more particularly to a method for achieving a uniform axial and radial oxygen concentration of a silicon single crystal.
2. Statement of the Related Art
When a silicon single crystal is prepared according to the Czochralski process, polycrystalline silicon as a material contained in a quartz crucible is heated to melt and form a silicon melt, into which a seed crystal is immersed, whereby the seed crystal is drawn slowly to grow a single crystal silicon rod. The surface of the quartz crucible in contact with the silicon melt (approximately 1,450.degree. C.) is dissolved and evaporated in part as silicon monoxide from the surface of the melt. The balance of the dissolved surface is incorporated into the growing silicon single crystal. In general, the oxygen concentration distribution of the silicon single crystal pulled according to the Czochralski process is not uniform in the bulk, that is, axially higher at the portion solidified initially and lower at the portion solidified later. For example, the oxygen concentration of the portion solidified initially is 3.times.10.sup.18 atoms/cc of oxygen, while that of the tail end portion of the pulled single crystal rod is exceedingly reduced to 6.times.10.sup.17 atoms/cc. Radially, on the other hand, the concentration is higher at the center and lower at the periphery. The ratio of variation in oxygen concentration from center to periphery amounts to 15%.
Oxygen dissolved in silicon single crystal forms microdefects when subjected to heat treatment. The presence of dissolved oxygen in the active layer region of the surface of the semiconductor crystal substrate causes the maknig of nuclei which originate stacking faults during thermal oxidation of the surface, and which have an unfavorable influence on fundamental characteristics of semiconductor devices. In recent years, however, such microdefects are forced to be generated in the bulk of a semiconductor substrate so that the microdefects can be positively used only as a gettering center of impurities. In addition, to positively use of the microdefects, a technology has been developed whereby the active layer region is made defect-free with the help of gettering effect of the microdefects in the bulk. Therefore, oxygen of solid solution is indispensable now. Such a treatment is called an intrinsic gettering process which is an inevitable technology for semiconductor integrated circuits.
In this sense, it is required that the concentration of oxygen dissolved in pulled silicon single crystal is as uniform as possible both axially and radially. As such technologies for controlling oxygen concentration distribution in a silicon single-crystal rod there are the following:
(1) A method wherein the rotation of a quartz crucible is brought to a complete stop periodically to make use of fluid friction in the vicinity of the solid-liquid interface (Japanese Patent Examined Publication No. 53-29677).
(2) A method wherein an oxygen concentration profile in a silicon single-crystal rod is measured to control the rotation rate of the crucible so that the rotation will be varied in a reverse relationship with the measured profile (Japanes.e Patent Examined Publication No. 60-6911).
(3) A method wherein a silicon single-crystal rod and a quartz crucible rotate in a reverse direction with each other. The silicon single-crystal rod rotates faster than the crucible. The rotation rate of the quartz crucible increases with the length of the silicon rod increasing in growth (Japanese Patent Laid-Open Publication No. 57-135796). None of them are satisfactory.
According to the method (1), it is necessary that the quartz crucible should be completely suspended momentarily. The large volume of the melt contained in the crucible in accordance with the recent trend where the diameters of the quartz crucibles are large makes it hard physically to stop the crucible abruptly because of the mass of the melt and the mechanical structure of a driving means. A momentary complete suspension of the crucible, if possible, would produce a thermal instability of the melt within the quartz crucible. As a result, an unfavorable solidification of the melt in part at the bottom of the crucible would follow. Further, such a mere momentary complete stop of the crucible would not improve an axial oxygen concentration profile.
According to the method (2), when the oxygen concentration is reducing along the length of the silicon rod, the rotation rate of the crucible is caused to increase. This method is not practical, since a complicated procedure will follow in determining a suitable rate of rotation of the quartz crucible. Furthermore, this method would not improve the radial oxygen concentration profile.
The method (3) resembles the method (2). If attention is paid only to the rotation of the quartz crucible, this method is equivalent to a case where the oxygen concentration profile in a silicon single crystal reduces lengthwise uniformly. According to this method, the rotation of the crystal is selected to be in a reverse direction relative to the rotation of the quartz crucible, and further, programmed to be progressively faster. This method, however, does not necessarily improve the radial oxygen concentration distribution.